Flow stoppage detector



Jan. 30, 1968 R. A. DEANE FLOW STOPPAGE DETECTOR Filed July 21, 1966R0552? A. DEA v5 IN VENTOQ ATTORNEY United States Patent 3,366,942 FLOWSTGPPAGE DETECTOR Robert A. Deane, 22344 Maudell St, Canoga Park, Calif.91304 Continuation-in-part of application Ser. No. 466,067,

June 22, 1965. This application July 21, 1966, Ser.

6 Claims. (til. 340-243) ABSTRACT OF THE DISCLOSURE A device fordetecting the stoppage of flow in a fiuid comprising a heater, a firstand second heat sensor and means for detecting differential heatresponses between the sensors, said heater and sensors being thermallyconnected through their bases, immersed in the fluid, positioned topermit the unobstructed flow of the fluid between the heater and thesecond sensor, and adapted so that when the fluid is flowing the heatgenerated by the heater and passing into the fluid is dissipated withoutheating either of the sensors and when the fluid is at rest said heatheats the second sensor through the fluid to a greater degree than thefirst sensor.

This application is a continuation-in-part of my prior copendingapplication Ser. No. 466,007, filed June 22, 1965, now abandoned.

The subject invention relates to flow stoppage detectors, andparticularly to detectors of the type employing thermodynamic andfluidic principles for selectively sensing the cessation of flow ofmaterials in fluid, gaseous or solid form.

In many industrial and commercial fields there is a great and heretoforeunsatisfied need for a compact and versatile flow detector whichpositively determines that a particular mass of material has stoppedflowing. This need is perhaps most stron ly felt in the petroleum andcommercial gas industries, which use hundreds of thousands of miles ofpipes, tubes, ducts, and other conduits to transport, and countlesstanks to store, enormous volumes of material in a variety of forms.These conduits and containers may be vertical, horizontal or inclined,and range in size from fractions of inches to many feet in diameter. Thematerials vary in composition from gases and highly reactivelow-viscosity liquids to semi-solid completely heterogeneous corrosivemixtures of sand, mud, water, and crude oil.

Frequently the conduit or storage vessel contains several diflerentmaterials having widely diifering physical and chemical properties.These materials may be intermixed in a heterogeneous mass or they mayform a number of fairly well defined strata.

Often the transport lines and tanks and the pumping equipment associatedwith them are unattended for long periods. Failure to note a halt in theflow of one or more of the materials in them is almost always costly,and may be catastrophically dangerous. Economic inflation and advancesin technology have made, and will continue to make such failures evermore costly and dangerous.

Devices have long been available for detecting, and in some casesmeasuring the rate of flow of fluids. The most common of these utilizethe force exerted by the moving fluid itself against some objectimmersed in it to indicate or determine the rate of fluid motion.Regardless of the form chosen for the immersed object, for examplepropeller, vane, piston, deflection arm, drogue, or the like, all ofthese devices are subject to a number of serious shortcomings. Moveableparts deteriorate after continued immersion for extended periods andbecome corroded or 3,35,942 Patented Jan. 30, 1968 frozen in place aftereven brief contact with many fluids. Sealing and packing, always atleast minor problems, become monumental tasks where moving parts areinvolved. Clogging, jamming and fouling frequently occur where the fluidcontains any solids, tars or lacquers, or forms them through chemicalreaction or chemical decomposition. Mechanical deformation and fatigueinduced breakdowns also plague this class of indicators. For all of theforegoing reasons, and in addition because their response rates andsensitivities in fluids of high density and viscosity are generallyextremely low, particularly when these fluids are moving slowly, thesedevices are by and large wholly unsuitable for the detection of flowstoppage.

Another family of flow sensing devices operates on the Venturiprinciple; but these are wholly unsatisfactory for use with very denseor slow moving fluids. Furthermore, when the fluid is of high viscosityor contains solids there is little chance of keeping their orifices,manometer tubes, bellows, and other pressure sensing or conductingmechanisms free; and they are quickly rendered inoperative. Even whenoperating properly these devices are unable to indicate positively thetermination of fluid flow because all of the above environmental factorsinfluence the delicately balanced signals near and at the zero flowrate.

A completely distinct family of flowmeters described broadly as theThomas type and operating on the electrocaloric principle has beendeveloped in an attempt to overcome many of the deficiencies associatedwith the mechanical and Venturi types of detectors and flow meters.Basically, the electrocaloric type meter measures flow rate bydetermining the effect of a given amount of heat dissipated into astream of fluid flowing in a conduit. Typical examples of this type offlow meter are the Laub flow meters illustrated in United States PatentsNos. 2,729,- 976, 2,953,022, and 2,972,885, and the Howland, Davis,Brion, Morgan, Hathaway, Skibitzke, and Adams flowmeters illustrated inUnited States Patents Nos. 3,196,679, 3,030,806, 2,548,939, 2,580,182,2,647,401, 2,728,225, and 2,859,617, respectively. Although thesedevices have indeed avoided or eliminated many of the defects mentionedearlier, they suffer from their own inherent deficiencies.

The Laub flowmeters require that the fluid be passing or be divertedthrough a relatively narrow conduit or pipe line, and are not readilyadaptable to detect the motion of fluids in large ducts or opencontainers. The Laub installations rely on the ability of a temperatureresponsive coil surrounding the closed conduit to respond to an increasein the temperature of the conduit wall resulting from the transfer ofheat from the fluid to the wall. Heat is added to the fluid by means ofa second coil surrounding the conduit upstream of the first heat sensingcoil, and is carried to the downstream coil by forced convection. Thisarrangement measures only the flow of the boundary layer of the fluid inthe conduit, and is totally useless with fluids having very denseboundary layers and with non-homogeneous fluids. Also, it will notperform properly where there is substantial turbulence in the fluid pathor when the fluid is flowing at a very high or a very low rate, in theformer case because of insufficient heating of the fluid, and in thelatter because of the excessive dissipation of heat both within thefluid and into the conduit wall. Furthermore, and for many purposes ofgreatest importance, the Laub system does not furnish a positive meansof determining that all flow has ceased.

The Howland, Davis, Brion, Morgan, Hathaway, Skibitzke, and Adamsdevices likewise have several very significant drawbacks. Probably themost significant of these is that each of them relies on forcedconvection, i.e., the motion of the fluid itself, for the transfer ofheat from a heating element to a temperature responsive sensor. Whilethe forced convection principle may be of value for use in instrumentsfor measuring moderate rates of how of fluids through conduits, it iswholly inadequate for detector applications intended to give a reliableindication when all motion in the fluid has ceased.

By their very nature, all of these devices, like the Laub invention, arehighly susceptible to vagaries in the composition and physical andchemical characteristics of the materialand the condition of thematerial, such as its temperature and stratification. And in addition,the Laub fiowmeter is strongly affected by ambient air temperaturechanges as well. It should be noted also that neither the Laub" nor anyof the other named devices will operate properly where the fluid path isinclined more than slightly upwardly or downwardly, as may be the casein those very situations in which fiow stoppage detection is vitallyneeded in the field.

A particularly serious deficiency characteristic of all of the abovementioned devices is their inability to function properly in thepresence of heterogeneous mixtures of materials or in situations inwhich the material or materials appear in several physical states. Thisdeficiency is magnified where the materials are stratified, as where apipeline contains solidifying tar near its bottom, one or more layers ofliquid above the tar and gases above the liquid surface. Unfortunately,it is often of great importance to be able to discriminate between suchstrata and detect flow stoppage in one of them regardless of thepresence or absence of flow in the others.

For these and many other reasons, not only a new instrument but also anew approach to the solution of the problems cited is necessary. Thesubject invention is such an instrument and embodies such an approach.

The subject invention has many objects, all of them directed toward theprovision of a flow stoppage detector which eliminates, or substantiallyreduces the effect of all of the foregoing deficiencies. These objectsinclude providing:

A simple, yet durably constructed device having no moving parts forpositively and reliably detecting the stoppage of movement of a flowingmedium;

Such a device capable of performing in any medium, regardless of itsphysical and chemical form, characteristics and composition, and ofcontinuing to function even while its form, characteristics andcomposition are changing;

A device of this character which will operate equally well in a closedconduit, a covered vessel, or an open container;

A flow stoppage detector which can be applied to existing lines, vesselsand containers without costly refitting, and which can be readilyreplaced without removing the conduit, vessel or container fromoperation; and

A detector which can be used where the flow of the medium is nothorizontal.

Another and particularly significant object is the provision of a flowstoppage detector which is capable at once of utilizing the principlesof both convective and conductive heat transfer, and of utilizing one orthe other or both in the presence of materials in one or more physicalstates and in one or more strata.

Other objects and advantages will become apparent as the subjectinvention and its operation are described by reference to theaccompanying drawings, in which:

FIGURE 1 is a top plan view looking downward at a length of fluidtransmission line having a portion of its upper wall cut away to showone preferred form of the subject invention in place;

FIGURE 2 is a longitudinal vertical sectional view of part of the lineillustrated in FIGURE 1 taken in the direction 2-2;

FIGURE 2a is a fragmentary top plan view of a modification form of thesubject invention;

FIGURE 3 is a sectional view similar to that of FIG- URE 2 illustratinga modified form of the subject invention;

FIGURE 4 is a sectional view similar to that of FIG- URE 3 illustratinganother modified form of the subject invention in place in a verticalsegment of transmission line; and

FIGURE 5 is a simplified circuit diagram illustrating the basicelectrical circuit of the subject invention.

Referring now to the drawings, FIGURES 1 and 2 depict a segment of atypical fluid transmission line 11, such as a pipe line used forcarrying materials such as crude oil together with its accompanyinglight gases, saline water, sand, mud, waxes, tars and other impurities,including solids, from a producing oil well to a fieldside storage tank.The direction of flow is from right to left, as shown by the arrow. Forconvenience, and where practicable to do so, a prefabricated removeablesection of pipe 12, adapted by the provision on its side wall of aninternally threaded annular boss 13 and an access port in the wall toreceive the threaded head 18 of the detector, may be included in thepipeline. This arrangement, while desirable, is not necessary since theexisting lines may be adapted quickly and easily to receive the detectorby cutting or drilling the port and clamping or welding the preformedboss 13 around it.

The detector itself is preferably contained in a sturdy case having abody or base 14 and head 18. Projecting rigidly outwardly from base 14and extending through and beyond head 18 are elongated reference probe15, detector probe 16, and heater 17. As illustrated in FIG- URE 2, inone preferred arrangement with the detector in place and firmly attachedto the line 11, the longitudinal central axes of the portions of probes15 and 16 immersed in the materials within the line lie in substantiallythe same horizontal plane, with reference probe 15 upstream of detectorprobe 16 and heater 17. The portion of heater 17 within the line ispositioned vertically below the detector probe 16, and somewhat closerto detector probe 16 than to reference probe 15. The requirements of theinvention permit the probes 15 and 16 and the heater 17 to be positionedso as to have a very low overall profile, thus allowing them to beinserted into the line near its bottom. This capability is ofconsiderable advantage for installations in which very small quantitiesof material may be flowing along the bottom of the line.

Probes 15 and 16 are preferably conventional thermistors or similartemperature responsive sensors having a high negative temperaturecoefficient of resistance, hermetically sealed within and in thermalcontact with the inner walls of, cylindrical casings. These casings maybe made of any suitable material demonstrating high thermalconductivity, such as copper or aluminum.

It is to be understood that although the thermistor offers certainadvantages in terms of durability, simplicity of construction andcircuitry, and reliability, the subject invention contemplates the useof any convenient type of temperature responsive device, including thosesuch as thermocouples or thermopiles, for probes 15 and 16. If othersensors are employed it may be necessary to make some mlnormodifications in the electrical circuit shown in FIGURE 5 and discussedlater herein; but such modifications will not carry the detector outsidethe broad scope and fundamental concept of the subject invention.

Heater 17 comprises a conventional alternating or direct current heatingelement of any convenient form which, like probes 15 and 16 ispreferably contained in a thermally conductive cylindrical casing.

Preferably the casings of heater 17 and probes 15 and 16 are permanentlysecured at their upper ends to the inner wall of section 12 with theirbores open upwardly so as to be exposed when base 14 and head 18 areremoved from the line. By mounting probes 15 and 16 and heater 17 as aunit for swivelable connection to base 14 and head 18, the entiredetector unit may be adapted to be removed from the line and replacedwithout halting the flow of material and without the loss of any 0f thelines contents.

In one embodiment of the invention the portions of probes 15 and 16within head 18 and base 14 are thermally insulated from the base ofheater 17 and from each other, in order to insure that any heat sensedby the probes will be that of the particular material in which they areimmersed. The identical result is achieved in the embodiment of theinvention illustrated in FIGURE 2a by making head 18 of a thermallyconductive matter and mounting probe 15, of the same length as probe 16,at a point on head 18 the same distance from the mounting point ofheater 17, as probe 16, but angled away from heater 17 In a preferableembodiment, however, and one which constitutes a significant improvementover all of the prior art devices referred to earlier, means areprovided within head 18 or base 14 for the transfer of heat from heater17 to probes 15 and 16 along differential paths. This may beaccomplished by making the base 18 of FIG- URE 2 thermally conductive,or by connecting each of the probes to the heater by means of athermally conductive shunt (not shown). The desired heat pathdifferential may be achieved by attaching sensor probe 16 to themounting plate or head 18 at a point closer to heater 17 than the pointof attachment of reference probe 15, or by attaching both of the probesat substantially the same distance from heater 17 and having referenceprobe 16 projecting perpendicular to the mounting plate, with ref erenceprobe 15, of a greater length than probe 16, mounted to projectangularly away from heater 17.

Regardless of the specific means used, the purpose of this preferredarrangement is to insure that in the absence of other influences actingupon the two probes, probe 16 with its shorter heat transfer path fromheater 17 will experience greater heating than will probe 15.

The description of the operation of this preferred form of a detectorcan best be followed when considered along with a description of theelectrical circuit of the invention as shown in FIGURE 5.

This circuit may be miniaturized and contained wholly within thedetector casing, or its principal elements may be maintained at a sitesome distance from the detector.

Preferably, however, the detector is fully self-contained,

requiring no external electrical connections other than that to itssource of power 31.

Basically the circuit comprises a power source 31, which may be aconventional source of standard 110 volt alternating current or, withappropriate minor changes in circuit design, direct current; a commonline isolation transformer 32; and a typical rectifier 33 of the vacuumtube or gas filled type or, preferably, a semiconductor diode type,which, acting through resistor 34, serves to rectify the high frequencypulses and thereby charge condenser 35 with a fixed polarity. AWheatstone bridge is formed in the circuit and includes the thermistorsof probes 15 and 16 balanced against fixed resistance 39 and variableresistance 41, respectively. A voltmeter 37, galvanometer, ohmmeter, orother suitable current detecting or measuring device may be connectedacross .the bridge to give a visual indication when the bridge becomesunbalanced, or across thermistor 15 to measure directly the temperatureof the material in which it is immersed. In addition, if desired a relayswitch 38 likewise may be connected across the bridge. Leads 19 fromtherelay switch 38 may be connected to any desired auxiliary warningdevice, such as a light or alarm, or to a secondary operational circuit,such as one activating a standby pump or automated valve system or thelike. A separate source of current for heater 17 is shown; however,source 31 may be used for this heater with little or no adaptation.

In operation with the detector in place in transmission line 11 and withthe material flowing the detector circuit is connected to the powersources 31 and 36 and the Wheatstone bridge balanced by means ofvariable resistance 41. By selecting the differential heat transfermeans within base 18 to have a thermal conductivity lower than that ofthe flowing material to be monitored, the effect of heat transfer byconduction from heater 17 through base 18 and into probes 15 and 16 canbe effectively disregarded, since the heat thus transferred will bedissipated by convection through the flowing material before it reachesthe thermistors within the probes. Further balancing of the bridgeshould not be necessary unless the current flow rate in the line 11, 12or the material temperature changes radically.

Although heater 17 is continuously emitting heat, neither the initialnor the continuing balance between probe 15 and probe 16 is influencedby the emission since the heat is continuously being carried from bothprobes by forced convection current created by the flow of fluid. Thisis true whether the flowing medium be a finely divided solid, a lowviscosity liquid, a gas under low pressure, or a virtual solid.

Should the flow stop, where the fiuid is of relatively low viscosity,normal convection currents formed above the heater 17 immediately beginto rise vertically. The distance between the sensing portion ofreference probe 15 and heater 17 and the relative positions of the twoare carefully adjusted to insure that the sensor of probe 15 isuntouched by the vertically rising currents. The temperature ofreference probe 15 is thus not changed. The detector probe 16, locatedvertically above the heater 17, lies directly in the path of the risingnatural convection currents however, and bathed in the relative heat ofthese currents, its temperature and thus resistance rise, therebyupsetting the voltage balance of the Wheatstone bridge and givingpositive indication that the flow has stopped.

In the event the character of the medium will not support normalconvection currents, as in the case of semisolids or crystallized orsolidified media, heat is carried from heater 17 by normal thermalconduction. Since the sensor of detecting pro-be 16 is positioned closerto heater 17 than the sensor of reference probe 15, a greater quantityof heat carried by such conduction will reach the former, again throwingthe Wheatstone bridge out of balance and warning of the flow stoppage.Experimentation has demonstrated that heat transferred by radiation fromheater 17 is so insignificant that its effect on the operation of thedetector may be completely disregarded. This is particularly the case inthermal-opaque liquids or solids. v

A significant feature of the subject invention, and one whichdistinguishes it from all of the prior art devices mentioned earlier,resides in the fact that since it relies on normal convection currentsand normal conduction to indicate flow stoppage, and since such currentsand conduction become effective only when the fluid flow completelyceases, a sharp and clearly discernible line exists between theindications given by this device while there is any motion in the fluidand those given when the motion stops. With all of the prior flowmetersindications of fluid motion are given continuously or intermittently,even while the flow is subsiding, and the cessation of flow can beassumed only when the indications cease. The difficulty of ascertainingwith any degree of accuracy the moment at which such indicationsactually cease is obvious. In the subject invention this problem iscompletely obviated, because its indication of flow stoppage is not onlya sharp one, but in fact an increasing one.

With the fluid flow halted, the longer the natural convection currentsor normal conduction are allowed to continue the greater the temperaturedifferential between probes 15 and 16, and the further out of balancethe Wheatstone bridge circuit. By providing a second relay or auxiliarywarning device, set to be activated only when the bridge imbalancereaches a predetermined level appreciably higher than that at which thefirst relay or warning device is actuated, this characteristic of thesubject invention may be utilized to great advantage.

It will be observed that while probes and 16 are immersed in a flowingliquid medium or a moving solid medium the lossof heat transferred fromheater 17 through the bases of probes 15 and 16 within head 18 allowsthe system to operate precisely as if the probes were thermallyinsulated from heater 17 at their bases. When the flow of the liquid orsolid material ceases, however, the addition of heat to probe 16 throughits base as well as through the medium itself, either by convection orconduction, greatly amplifies the imbalance of the Wheatstone bridge andthe resultant flow stoppage warning. Similarly, since the heat absorbingcapabilities of gases are markedly lower than those of either liquids orall but a fairly small number of solids, it is possible by adjusting thebridge circuit to enable the detector system to selectively ignore theinfluence of gas flow without changing its sensitivity to the stoppageof the fluid or solid material sought to be monitored. On the otherhand, if desired, the system may quite easily be adapted to sense theflow or stoppage of flow of gas without regard to the existence orrelative motion of liquid or solid materials simultaneously present.

A somewhat unusual situation exists in certain operations such as themonitoring of gas production wells. Here the flow of the gas itself maybe quite sporadic; and the only real concern is the formation of ice,which may block the line with potentially disastrous results. With aslight modification in the position of detector probe 16, the subjectinvention is ideally suited to detect this dangerous condition.

All that is necessary here is to locate probe 16 out of the path of theascending convection currents formed when the gas flow ceases, and toadjust the warning system to ignore the heating effect resultingfrom'th'e differential heat transfer from heater 17 through the bases ofprobes 15 and 16. In this configuration, no stoppage warning will begiven during any normal operations, including the complete stoppage ofgas flow. Should ice form in the line, however, even the relativelysmall influence of heat conducted from heater 17 through the ice tosensor 16 will be suflicient to trigger the warning mechanism.

FIGURES 3 and 4 illustrate two of the numerous embodiments the subjectinvention may take. The form shown in FIGURE 3 is particularly suitablefor use in situations where it is anticipated that the fluid may beflowing alternately in opposite directions. For this purpose anadditional heat sensing reference probe 21 is provided on the oppositeside of heater 17 from probe 15. Conventional selector switch means inthe leads from probes 15 and 21 to the Wheatstone bridge circuit permitprobe 15 to be used as the reference when fluid flow is from right toleft, and probe 21 to be used as the reference when the flow isreversed.

Under some circumstances a considerable advantage may be gained bylocating the reference probe 15 or 21 downstream from, and either on thesame level as, or slightly higher than, heater 17. In thisconfiguration, if the Wheatstone bridge is balanced while heater 17 ison and the forced convection currents associated with the moving fluidare warming the reference probe the effect of flow stoppage on thebalance of the bridge circuit will be greatly magnified. Since thesensitivity and accuracy of the subject invention and the magnitude ofits response to flow stoppage are directly related to the temperaturedifferential between reference probe 15 or 21 and detector probe 16, itis obvious that under most conditions it is preferable to position thereference probe within the anticipated convection stream from theheater.

The arrangement of the probes illustrates in FIGURES 2 and 3 will provesatisfactory for use in fluids flowing in horizontal or even invertically ascending or descending lines or paths, To utilize either ofthese forms where the fluid path is inclined from the horizontal, it ismerely necessary to rotate the base 14 until detector probe 16 liesdirectly above heater 17 When the direction of flow is verticallydownward, either the embodiment of FIGURE 2 or FIGURE 3 will performsatisfactorily. When the direction of flow is vertically upward a slightmodification may be desirable, since detector probe 16 would besubjected to the heating effects of forced convection as well as naturalconvection. Even under this condition, however, either of these twoforms would still be operative, because detector probe 16 will retainfar more heat in the absence of fluid flow around it than in thepresence of such flow. Accordingly, the imbalance of the Wheatstonebridge will be far greater when the fluid surrounding the probes isstopped than when it is moving.

Experimentation has demonstrated that for many oil field applicationsmany advantages are to be gained by installing the detector of thesubject invention in a vertical standing section of the oil transmissionline. The chief advantage to this arrangement is the elimination of allbut the most transitory stratification effects.

As shown in FIGURE 4, another embodiment of the invention provides forthe interposition of an inclined deflector 27 partially obstructing thefluid path between heater 26 and detector probe 25. By properlyadjusting the placement and angle and incline of deflector 27 withregard for the characteristic of the fluid in which the device is to beused, the forced convection associated with even the slowest movingfluid can be deflected past detector probe 25, thereby eliminating orsubstantially reducing the possibility of heating caused by forced,rather than natural convection. Placement of reference probe 24 adjacentto detector probe 25 and in the path of the deflected forced convectioncurrent will further insure that the detector will be sharply responsiveto fluid stoppage.

For descriptive purposes the subject invention has been illustrated asit might be used to detect the stoppage of flow of materials throughenclosed conduits. It should be apparent that each of these forms willfunction quite as well in materials held in open containers or innatural bodies such as rivers, streams, lakes or seas. It may bedesirable to alter the form of the base 14 and head 18 for immersion insuch fluid media; but these modifications would in no way alter theunderlying theory or practice of the subject invention.

It must be understood that the particular forms illustrated here havebeen chosen for demonstrative purposes only, and are not intended tolimit the scope of the invention. The metes and bounds of the inventionitself are set forth inthe claims which follow.

What is claimed is:

1. A device for detecting the stoppage of flow in a selected mediumstratum comprising:

first heat sensing means including an independent heat sensing elementhaving a high negative coefficient of electrical resistance, mounted onfixed first heat transmitting supporting means, immersed in said mediumstratum, and responsive to changes in the temperature thereof;

second heat sensing means including an independent heat sensing elementhaving a high negative coefficient of electrical resistance, spacedapart from said first heat sensing means, mounted on fixed second heattransmitting supporting means, immersed in said medium stratum, andresponsive to changes in the temperature thereof;

means for heating said medium, mounted on fixed third heat transmittingsupporting means, said heating means being immersed in said mediumspaced apart from said heat sensing means and at a greater distance fromsaid first heat sensing means than from said second heat sensing means,

positioned to permit the unobstructed flow of said medium between saidheating means and said second heat sensing means, and

positioned to have no greater heating effect through said medium oneither of said heat sensing means as long as said medium stratum is in astate of flow, and to have a greater heating effect through said mediumon said second heat sensing means than on said first heat sensing meanswhen the fiow of said medium stratum effectively ceases;

heat transmitting means in constant thermal communication between saidthird supporting means and said first supporting means and providingwith said first and third and said second and third supporting means afirst and second heat path, respectively; and means for detecting thedifierential in the temperature responses of said first and second heatsensing means resulting from the effective stoppage of said flow.

2. A device for detecting the stoppage of flow in a selected mediumstratum as defined in claim 1 in which said heating means is positionedunder said second heat sensing means.

3. A device for detecting the stoppage of flow in a selected mediumstratum as defined in claim 1, in which said first and second heat pathsare of equal length.

4. A device for detecting the stoppage of flow in a selected mediumstratum as defined in claim 1, in which said second heat path isappreciably shorter than said first heat path.

5. A device for detecting the stoppage of flow in a selected mediumstratum as defined in claim 1, adapted to be inserted through an openingin the wall of a conduit or container holding said medium, in which saidheat sensing elements and said heating means are adapted to be insertedremoveably into elongated hollow thermally conductive casings which areclosed at one of their ends and fixedly attached to the inside of saidwall with their closed ends projecting into said medium stratum andtheir open ends exposed through said opening in said wall, said ser1singelements and heating means being in thermal contact with the walls ofsaid casings adjacent their closed ends; and

the distance between the closed end of the casing containing said firstheat sensing means and the closed end of the casing containing saidheating means being appreciably greater than the distance between theclosed end of the casing containing said second sensing means and theclosed end of the casing containing said heating means.

6. A device for detecting the stoppage of flow in a selected mediumstratum as defined in claim 5, in which the casings containing saidfirst and second heat sensing means are attached to the inside of saidwall equidistant from the point of attachment of the casing containingsaid heating means, and are of equal length.

References Cited UNITED STATES PATENTS 2,736,784 2/1956 Gore 73-362 X2,799,758 7/1957 Hutchins 73-362 X 2,938,385 7/1960 Mack et al 73-3622,961,625 11/1960 Sion 73-362 X 3,030,806 4/1962 Davis 73-204 3,147,6189/1964 Benson 73-204 3,196,679 8/1965 Howland 73-204 FOREIGN PATENTS1,23 8,716 7/1960 France.

OTHER REFERENCES Thermistors, Their Characteristics and Uses, Pearson,in Bell Laboratories Record, 19 (4); pp. 106-111, December 1940.

Thermistors in Electric Circuits; R. R. Batcher, in ElectronicIndustries, 4 (1): pp. 7680, January 1945.

Thermal Resistor Elements For Electrical Circuit Applications, inProduct Engineering, R. S. Goodyear, 16 (2), pp. 93-95, February 1945.

RICHARD C. QUEISSER, Primary Examiner. JAMES J. GILL, Examiner.

I. C. GOLDSTEIN, E. D. GILHOOLY,

Assistant Examiners.

